1. Field of the Invention
The present invention relates to a ceramic resonance type electrostatic sensor apparatus for detecting a change in small capacitance of an object to be detected by using a high-frequency oscillation signal.
2. Description of the Prior Art
A generally used electrostatic sensor apparatus is designed to change the oscillation frequency of an oscillator by changing a capacitance, as an external capacitance, used in a tank circuit of the oscillator. However, the sensitivity of this sensor apparatus is low. For this reason, an apparatus having a higher sensitivity (designed to obtain an AM-modulated wave by changing the capacitance of a capacitor of a resonator having a resonance frequency slightly deviated from the oscillator frequency of an oscillator) has been used more often recently. An electrostatic sensor apparatus comprises an oscillator 1, a resonator 2, a detecting portion 3 for detecting a change in capacitance between the detecting portion and an object to be detected, a detector 4, and an amplifier 5, as shown in FIG. 1. The oscillator 1 and the resonator 2 respectively include resonator elements. The resonance frequency of the resonator element of the resonator 2 is modulated in accordance with a small change in capacitance detected by the detecting portion 3. The modulated resonance frequency is then extracted as an output signal from the resonator 2 and is detected and amplified.
In a widely used electrostatic sensor apparatus of this type, one of the resonator elements of the oscillator and the resonator 2 is constituted by a strip line. However, when a resonator element is to be constituted by a strip line, a strip line length must be 1/4 the wavelength of light. Since a long strip line is required, it is difficult to reduce the apparatus in size.
In order to reduce the size of the apparatus, Japanese Patent Laid-Open No. 58-85948 discloses an apparatus wherein the resonator elements of the oscillator 1 and the resonator 2 are constituted by dielectric resonator elements. If the resonator elements are constituted by dielectric elements, the length of each resonator element can be reduced to .epsilon..sup.-1/2 that of the resonator element constituted by a strip line. If this dielectric element is constituted by a ceramic element, since ceramics have dielectric constants .epsilon. of 20 to 40, the apparatus can be greatly reduced in size. In addition, if each dielectric element is constituted by a ceramic element, since the Q values of ceramics are as high as 200 to 300, the resonator element can be expected to have a higher sensitivity than the resonator element constituted by a strip line.
In an electrostatic sensor apparatus of this type, however, since the operating point of the resonator element of the resonator 2 is set at a point deviated from the resonance frequency of the resonator element of the oscillator 1, an impedance at this operating point is normally 200 to 500 .OMEGA. or more. If a load having an impedance lower than this impedance is coupled to a peripheral portion of the resonator element of the resonator 2, the resonator element is subjected to Q damping (the Q is decreased). That is, the ceramic resonator element cannot exhibit its original characteristics (that the Q is large), and hence the detection sensitivity cannot be greatly increased.
Furthermore, in the electrostatic sensor apparatus shown in FIG. 1, assume that the impedance or Q of an object to be detected is low, and ion components in, e.g., a human body or water are to be detected. In this case, if an object detecting electrode of the detecting portion 3 and the resonator are directly coupled to each other, the Q of the resonator element of the resonator 2 is considerably decreased due to the low impedance of the object. Therefore, in an electrostatic sensor apparatus whose output voltage depends on the large Q of such a resonator element, the detection performance is decreased due to a great decrease in detection output, and the apparatus cannot be used in practice.
In the electrostatic sensor apparatus shown in FIG. 1, if dielectric resonator elements, especially ceramic resonator elements, are used in place of the respective resonator elements of the oscillator 1 and the resonator 2, the apparatus can be reduced in size. In addition, the sensitivity of the apparatus can be greatly increased because of the high Q values (200 to 300) of ceramics. Therefore, the apparatus can detect a small capacitance of about 1.times.10.sup.-5 pF.
FIG. 2 shows the oscillator 1 as a characteristic feature of the electrostatic sensor apparatus in FIG. 1. In this apparatus, a trimmer capacitor 13 for finely adjusting the oscillation frequency is connected to a ceramic resonator element 11 constituting the oscillator 1.
In such a high-sensitivity electrostatic sensor apparatus, the tuning point of the oscillator 1 and the resonator 2 must be accurately set. The capacitance of the trimmer capacitor 13 is known to include a fixed capacitance component C.sub.0 and a variable capacitance component .DELTA.C. The fixed capacitance component C.sub.0 is considerably large and greatly varies in each product. Since the fixed capacitance component C.sub.0 also serves as an oscillator element as indicated by dotted lines in FIG. 2, when a very high oscillation frequency of 1 GHz to 10 GHz is to be used, the oscillation frequency of the oscillator 1 greatly fluctuates due to this variation in C.sub.0. Even if the variation in resonance frequency of the ceramic resonator element 11 is suppressed to the rated value or less, a predetermined oscillation frequency as a target frequency cannot be obtained. Therefore, even if the capacitance is adjusted by the variable capacitance component .DELTA.C, the oscillator 1 and the resonator 2 cannot be set at a predetermined resonance point.
Assume that the resonance point can be accurately set. Even in this case, if a stray distributed capacitance is produced between the electrostatic sensor apparatus and peripheral members when the apparatus is mounted on a unit to be measured, the oscillation frequency is disturbed.
Although the trimmer capacitor 13 is ideally mounted near the ceramic resonator element 11, the capacitor 13 is often required to be mounted at a position separated from the ceramic resonator element 11 due to limitations in terms of a user's requirement, a position of a unit to be measured at which the electrostatic sensor apparatus is mounted and the like. In such a case, the leads of the trimmer capacitor 13 are elongated, and portions L.sub.1 and L.sub.2 of the leads may become capacitance components or inductance components. These components may cause variation in oscillation frequency or parasitic vibrations, resulting in an unstable oscillation frequency.
Recently, a demand has arisen for parallel processing of detection signals based on capacitances detected by an electrostatic sensor apparatus in different forms, or more accurate analysis of detection signals. In order to achieve parallel processing of detection signals, a plurality of sensor circuit systems each constituted by the components from the oscillator 1 to the amplifier 5 may be adjacently arranged. In order to realize accurate signal analysis, at least two sensor circuit systems each constituted by the components from the oscillator 1 to the amplifier 5 may be formed, and a differential output of output signals of the respective systems may be obtained.
In the case wherein the plurality of censor circuits each constituted by the components from the oscillator 1 to the amplifier 5 are arranged adjacent to each other, if a small capacitance is to be detected especially at a high sensitivity of about 1.times.10.sup.-5 pF, it is difficult to match the oscillation frequencies of the respective systems because of the influences of external disturbances other than detection signals or a stray distributed capacitance produced when the electrostatic sensor apparatus is mounted in a unit to be measured. If the oscillation frequencies of the respective systems are deviated from each other even by a slight value, mutual interference such as resonance occurs between the oscillation frequencies of the respective systems. For example, if one the other has an oscillation frequency of f.sub.0 ', a beat frequency of f.sub.0 '-f.sub.0 or f.sub.0 '+f.sub.0 is generated due to mutual interference. This frequency acts as an external disturbance, and accurate signal processing cannot be performed.